With the callback we can check if the origin trial is turned on for a
given context.

I will not land the other CL which added a flag to the isolate. The
information if the origin trial is on is context-specific and not
isolate-specific, and it's hard on the embedder side to track all
creations of a context.

With the API proposed in this CL we will ask the embedder every time we
start compilation whether the origin trial is on or off.

R=yangguo@chromium.org

Bug:868844

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I8822f40ab12582a5b0bd6640790a269107fc085a
Reviewed-on: https://chromium-review.googlesource.com/1163621
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55011}